1. Field of the Invention
The present invention relates to a light emitting device formed of a material having a piezoelectric effect.
2. Description of the Background Art
Semiconductor light emitting devices such as semiconductor laser devices and light emitting diodes using III group nitride semiconductors such as GaN, GaInN, AlGaN, and AlGaInN (hereinafter referred to as nitride based semiconductors) have been expected to be applied as light emitting devices for emitting light in a visible-ultraviolet region.
Out of the applications, extensive development has been proceeding toward practical applications of the semiconductor light emitting devices having a GaInN quantum well layer as a light emitting layer. Such semiconductor light emitting devices have been fabricated on a (0001) surface of a substrate composed of sapphire or silicon carbide (SiC) by MOVPE (Metal Organic Vapor Phase Epitaxy) or MBE (Molecular Beam Epitaxy).
FIG. 42 is a schematic sectional view showing the structure of a conventional GaN based semiconductor light emitting device. The semiconductor light emitting device shown in FIG. 42 is disclosed in JP-A-6-268257.
In FIG. 42, a buffer layer 62 composed of GaN, an n-type contact layer 63 composed of n-GaN, a light emitting layer 64 having a multiple quantum well (MQW) structure, and a p-type cap layer 65 composed of p-GaN are formed in this order on a sapphire substrate 61. The light emitting layer 64 is constructed by alternately stacking a plurality of barrier layers 64a composed of GaInN and a plurality of quantum well layers 64b which differ in composition.
In a method of fabricating such a conventional semiconductor light emitting device, generally used as the sapphire substrate 61 is one having an approximate (0001) surface as its principal plane, to successively form the respective layers from the buffer layer 62 to the p-type cap layer 65 on the sapphire substrate 61. In this case, the respective layers from the n-type contact layer 63 to the p-type cap layer 65 are grown as crystals in a [0001] direction of a nitride based semiconductor.
In a crystal, having no center of symmetry, having a zinc-blende structure, a wurtzite structure, or the like, however, a piezoelectric effect may be generally generated by a strain. In the zinc-blende structure, for example, a piezoelectric effect is the greatest in a strain compressing or extending with respect to a [111] axis. In the wurtzite structure, the piezoelectric effect is the greatest in a strain compressing or extending with respect to a [0001] axis.
In the conventional semiconductor light emitting device, the light emitting layer 64 composed of GaInN has a quantum well structure having a (0001) plane as its principal plane. The lattice constant of a quantum well layer 64b composed of GaInN is larger than the lattice constant of the n-type contact layer 63 composed of n-GaN. Accordingly, a compressive strain is exerted on the quantum well layer 64b in an in-plane direction (a direction parallel to an interface) of a quantum well, and a tensile strain is exerted in a direction of confinement in the quantum well (a direction perpendicular to the interface). As a result, the piezoelectric effect generates a potential gradient in the quantum well layer 64b, so that a potential is low in the [0001] direction, while being high in a [000 1] direction. An energy band in the light emitting layer 64 having a quantum well structure in this case is illustrated in FIG. 43. FIG. 43 illustrates five barrier layers 64a and four quantum well layers 64b. 
As shown in FIG. 43, the potential gradient exists in the quantum well layer 64b in the light emitting layer 64. Accordingly, electrons and holes which are injected as current are spatially separated from each other, as shown in FIG. 44. As a result, in the semiconductor light emitting device, luminous efficiency is reduced. Particularly in the semiconductor laser device, threshold current is raised.
When impurities are added to the quantum well layer 64b in the light emitting layer 64, the effect of decreasing the potential gradient by the movement of carriers is produced. When both p-type impurities and n-type impurities are added to the quantum well layer 64b, however, the carriers are compensated for, so that the carrier concentration is lowered. Consequently, the effect of decreasing the potential gradient by the movement of the carriers is reduced. Particularly when the respective concentrations of the p-type impurities and the n-type impurities which are added to the quantum well layer 64b are approximately equal to each other, the effect of decreasing the potential gradient by the movement of the carriers is further lowered.
Such a phenomenon also occurs in a III–V group compound semiconductor (for example, a GaInP based semiconductor, a GaAs based semiconductor, or an InP based semiconductor) other than the zinc-blende structure and the wurtzite structure, a II–VI group semiconductor, and an I–VII group semiconductor. Particularly in the nitride based semiconductor, the piezoelectric effect is great. Accordingly, the potential gradient generated by the piezoelectric effect is increased, so that the drop in luminous efficiency and the rises in threshold current and operating current become significant.